Orthogonal frequency division multiplexing (OFDM) is a popular digital modulation scheme widely adopted by various modern communications systems such as LTE/LTE-A standard and IEEE 802.11 standard for wireless local area network (WLAN). In OFDM, the available radio frequency (RF) band is divided into multiple subcarriers (or equivalently, tones), and each subcarrier is independently modulated with a same or different digital modulation schemes such as M-ary Phase-Shift Keying (MPSK) and Quadrature Amplitude Modulation (QAM).
To generate a baseband OFDM symbol, a sequence of N complex symbols, S={Sk}, k=0, 1, . . . , N−1, which is a sequence of complex numbers that represent a data sequence in the frequency-domain, is to be processed as input. An inverse discrete Fourier transform (IDFT) operation is performed on the complex frequency sequence to generate a time-domain sequence of N complex symbols, s={sn}, n=0, 1, . . . , N−1.
Each time-domain number of sn is a complex number and can be expressed as sn=an+jbn, where an and bn, both being real numbers, are the in-phase (I) and quadrature-phase (Q) components of sn respectively.
After a series of processes performed by the transmitter and the receiver and propagation, the ratio between in-phase and quadrature-phase components of sn may change in the complex time sequence received for demodulation, which is generally referred to as I-Q imbalance. I-Q imbalance is mainly introduced by the up-converter and the RF power amplifier in transmitter, as well as the down-converter in the receiver, due to unbalanced gains between the in-phase and quadrature-phase branches.
Previous efforts to address I-Q imbalance generally focus on improving the accuracy during manufacturing of the related RF devices, such as RF power amplifiers, up-converters, and down-converters. Highly accurate RF devices are more expensive, and the resulting improvement I-Q imbalance that is achieved by the increased accuracy is limited.
Improvements OFDM systems to increase robustness to I-Q imbalance are desired.